In prior U.S. Pat. No. 4,056,465 a wastewater treating process is described in which the mixed liquor is initially formed by admixture of wastewater influent with recycled sludge in an anaerobic zone and such mixed liquor is thereafter passed to an oxic zone where it is subjected to oxidizing treatment. The oxidized mixed liquor is subsequently passed to a settling zone from whence a portion of the activated biomass is recycled to the initial anaerobic zone. The process of the patent is particularly applicable for treatment of phosphate-containing wastewater influent. By the initial treatment of the mixed liquor under anaerobic conditions as described in said patent, there is a selective production of nonfilamentous microorganisms capable of sorbing BOD under both anaerobic and oxidizing conditions and also capable of accumulating phosphates under oxidizing conditions. When the anaerobically treated mixed liquor is subsequently subjected to treatment with oxygen-containing gas under oxic conditions, the BOD therein is oxidized while phosphates are caused to accumulate in the solid biomass components. A portion of this biomass, separated from the oxidized liquor in a subsequent setting operation, is recycled to the anaerobic treating zone to provide the desired active biomass for initial admixture with wastewater influent. By maintaining the initial anaerobic treating zone substantially free of NO.sub.x.sup.31 (i.e. less than about 0.3 ppm and preferably less than about 0.2 ppm expressed as elemental nitrogen) and the dissolved oxygen content of the mixed liquor in that zone at below 0.7 ppm and preferably below 0.4 ppm, the selective production of non-filamentous microorganisms capable of sorbing BOD is promoted, which micro-organisms are also capable of storing polyphosphates under the oxidizing conditions subsequently encountered in the oxic treating zone.
The aforesaid U.S. Pat. No. 4,056,465 also describes a variation of the basic process wherein the mixed liquor is passed from the anaerobic zone to an anoxic zone (i.e. one containing NO.sub.x.sup.31 initially at least about 0.5 ppm but less than 0.7 ppm and preferably less than 0.4 ppm dissolved oxygen), positioned intermediate the anaerobic and oxic zones. In this modification there is an internal recycle of mixed liquor from the oxic zone back to the anoxic zone, thus providing the NO.sub.x.sup.- in the anoxic zone. The alternative modification including the intermediate anoxic treatment of the mixed liquor, is employed for effecting denitrification of the mixed liquor.
As explained in U.S. Pat. No. 4,271,026, the removal of phosphorous from wastewater in traditional oxygenating systems has not always been consistent, i.e. sometimes it worked and sometimes it did not. In all such prior art processes an extensive treating period is required ranging upwards of 15 to 20 hours or more. While in operations carried out under the conditions advocated in aforesaid U.S. Pat. No. 4,056,465, phosphate removal from the wastewater influent was always present, the extent or rate of phosphorus removal varied significantly, and there was no means provided for controlling the process to insure enhanced phosphorus removal, high rate processing and a basis for optimizing design of the system. The U.S. Pat. No. 4,271,026 discloses operating conditions under which enhanced phosphorus removal is better assured at adequately high rate process operations. This is accomplished, according to the U.S. Pat. No. 4,271,026, by maintaining a particular set of interrelated operating conditions within a specific envelope in the type of process wherein recycled activated biomass is mixed with a wastewater influent containing phosphate and BOD under anaerobic conditions, thereby promoting selective production of the desired type of non-filamentous microorganisms (capable of storing phosphorus as phosphates under oxidizing conditions). In a subsequent oxic zone the mixed liquor is treated with oxygen-containing gas to oxidize BOD and to effect storage of phosphate in the biomass. As in the previous U.S. Pat. No. 4,056,465, the anaerobic treating zone is maintained substantially free of NO.sub.x.sup.- or other oxidizing agents and the dissolved oxygen level anywhere in the zone at less than about 0.7 ppm.; the subsequent oxygenation of the mixed liquor is carried out in the oxic zone at a dissolved oxygen level of at least and preferably above 1 ppm anywhere in the zone. To obtain the desired and assured enhancement in phosphorus removal, the U.S. Pat. No. 4,271,026 advocates that there be maintained throughout the treating stages of the process (up to but not including the separation of supernatant liquor from settled biomass) a BOD/P ratio from about 5:1 and up to about 50:1 and an F/M ratio from about 0.09 up to an upper limit of about 1.4.
In pending patent application Ser. No. 282,674 filed July 13, 1981 further improvements are disclosed applicable to wastewater treating systems of the type hereinabove described, by which substantial savings in operating costs can be achieved with effective BOD removal from wastewater influent, while obtaining dense sludge of good settling characteristics and desired high to adequate removal of phosphate. The cost savings stem chiefly from modifications achieving lower oxygen consumption and lowered power requirements for oxygen mass transfer as compared to conventional wholly aerobic activated sludge systems or other prior art systems, such as of the type disclosed in U.S. Pat. No. 4,056,465, which employ one or more anaerobic stages in conjunction with one or more aerobic stages. The operating conditions advocated in said pending patent application include (1) initial mixing of the recycled biomass with wastewater influent in a BOD sorption zone under selected conditions such that less than 5% of the total BOD.sub.5 is oxidized by oxygen or other agents while at least 25% of the soluble BOD becomes sorbed by the biomass; (2) subsequently subjecting the mixed liquor to controlled aeration conditions so that the dissolved oxygen level remains below 1 ppm and whereby at least 30 % of the total BOD.sub.5 separate out the more dense sludge, a portion of which is recycled to the initial BOD absorption zone.